Field of the Invention
This invention relates to a method of making a poly-crystalline silicon semiconductor film from an amorphous silicon film and, more particularly, to a method of manufacturing a poly-crystalline silicon film in which laser pulses are applied to anneal an amorphous silicon film deposited on a glass substrate while the substrate or the laser beams are moved in a predetermined direction. The poly-crystalline silicon (p-Si) film is suitable for thin film transistors (TFTs), such as pixel switching and driver circuit TFTs used for a liquid crystal display device.
At the present, active-matrix type liquid crystal display devices are mass-produced. Such liquid crystal display devices, however, include amorphous silicon (a-Si) insulation-gate type TFTs. Since the mobility of electrons under electric fields (.mu.FE) in a-Si TFTs is equal to or lower than 1 cm.sup.2 /Vs, the a-Si TFTs are not sufficient in capability for high resolution, high speed and high performance display devices. Poly-crystalline silicon (p-Si) TFTs, on the other hand, have been experimentally made by means of a laser annealing process in which excimer laser pulses are applied to anneal a-Si films and make the same into p-Si films for TFTS. The mobility of electrons under electric fields in such p-Si TFTs ranges from 100 cm.sup.2 /Vs to 200 cm.sup.2 /Vs. The p-Si TFTs are expected to be essential components for high resolution, high speed and sophisticated function (e.g., driver-circuit-integrated) display devices.
Such an annealing process is carried out by laser annealing equipment 50 shown in FIG. 4. The equipment 50 includes an excimer laser generator 51, an optical system module 52, an annealing chamber 54, a control apparatus 55, a substrate cassette station 56 and a manupulation robot 57. The excimer laser generator 51 generates a XeCl gas excimer laser 53 with the wavelength of 308 nm, for instance. As shown in FIG. 5, the laser 53 is applied to an a-Si film substrate 62 through a reflection mirror 61. The substrate 62 is moved in the direction 63 at a regular speed. The laser pulse beam size on the a-Si surface is 200 mm long and 0.4 mm wide, for instance. This pulse beam is oscillated at the frequency of 300 Hz. As a pulse applied region is gradually moved, the a-Si film in the region is successively poly-crystallized.
It is noted that a grain size (or diameter) of p-Si is a decisive factor of the mobility of electrons in p-Si TFTs under electric fields. The grain size mainly depends on energy density of the laser called energy fluence. The relationship between the grain size and the fluence is schematically shown in FIG. 6. Generally, the grain size becomes larger as the energy fluence is increased. However, the grain size is not changed even if the energy fluence is increased in value from F1 through F2. The grain size again becomes larger in response to the fluence increase in value from F2 through F3. The p-Si, however, is transformed into micro-crystalline silicon in the case where the fluence value is greater than F3. In such micro-crystalline silicon, the mobility of electrons under electric fields (.mu.FE) decreases so that the micro-crystalline silicon does not provide desired TFT characteristics.
A checking technique for the grain size is to etch the p-Si in Secco's solution and to observe the etched grain with a scanning electron microscope. This technique is used to properly set the fluence in the middle of the region between F1 and F2 where the grain size is not substantially changed. Laser oscillation intensities are always changed. Nevertheless, where the laser annealing is carried out in that region, a uniform grain size of p-Si can be obtained regardless of such laser oscillation intensity changes.
The inventors of the present invention have discovered that the fluence range between F1 and F2 is determined in accordance with a moving direction of a glass substrate relative to the shorter axis of the laser beams, and that a uniform grain size of p-Si cannot be obtained from a laser annealing process in which such a moving direction is not properly selected.
As described above, the laser beam is 200 mm long and 0.4 mm wide. Its profile is shown in FIG. 7 in which the X- and Y-axes represent the width of the laser beam and the energy fluence of the laser beams, respectively. The fluence distribution is not constant but is declined slightly as shown. The laser beam scanning direction +X or -X is a moving direction with respect to the substrate. The "+X scan" is to move the substrate from the lower energy fluence to the higher energy fluence along the X-axis. The "-X scan" is in the direction reverse to the "+X scan". The energy fluence is defined by an average value between the lower and higher values. The micro-crystal generation level energy fluence can be schematically defined as in FIG. 8.
FIG. 9 shows experimental data carried out by the inventors under the following conditions:
Substrate Size: 300 mm.times.400 mm PA1 Film Layer Structure: Glass Substrate/SiNx/a-Si PA1 Fluence Range: 380 mJ/cm.sup.2 through 430 mJ/cm.sup.2 PA1 Atmosphere: Nitrogen Gas with the Normal Atmosphere Pressure PA1 Laser Irradiation: 35 .mu.m Pitch (10-Time Irradiation) PA1 Laser Repetition Frequency: 300 Hz PA1 Substrate Temperature: Room Temperature PA1 Substrate Washing: No Washing before Laser Annealing
Quite importantly, the inventors have discovered the fact that the -X scan is wider in margin of the energy fluence to cause 0.3 .mu.m or larger average grain size of poly-crystalline silicon than the +X scan as illustrated in FIG. 9.